The invention relates to a method for determining an accurate time of transmission of a signal part of a code modulated signal transmitted by a beacon of a positioning system and received by a receiver of said positioning system, which receiver tracks beacons signals with a tracking unit. The invention relates equally to a corresponding receiver and to a positioning system comprising a receiver.
A well known positioning system which is based on the evaluation of signals transmitted by beacons is GPS (Global Positioning System). The constellation in GPS consists of more than 20 satellites employed as beacons that orbit the earth. The distribution of these satellites ensure that usually between five and eight satellites are visible from any point on the earth.
Each of the satellites, which are also called space vehicles (SV), transmits two microwave carrier signals. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) Code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s, which information comprises in particular ephemeris and almanac data. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position and the velocity of the satellite for any time while the satellite is in the respective described section. The orbits which are calculated based on ephemeris parameters are quite accurate, but the ephemeris parameters are only valid for a short time, i.e. for about 2-4 hours. The almanac data, in contrast, contain coarse orbit parameters. The orbits calculated based on almanac data are not as accurate as the orbits calculated based on ephemeris data, but they are valid for a longer time, a week or even more. Almanac and ephemeris data also comprise clock correction parameters which indicate the current deviation of the satellite clock versus a general GPS time.
Further, a time-of-week TOW count is reported every six seconds as another part of the navigation message.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and a tracking unit of the receiver detects and tracks the channels used by different satellites based on the different comprised C/A codes. The receiver first determines the time of transmission of the ranging code transmitted by each satellite. Usually, the estimated time of transmission is composed of two components. A first component is the TOW count extracted from the decoded navigation message in the signals from the satellite, which has a precision of six seconds. A second component is based on counting the epochs and chips from the time at which the bits indicating the TOW are received in the tracking unit of the receiver. The epoch and chip count provides the receiver with the milliseconds and sub-milliseconds of the time of transmission of specific received bits.
Based on the time of transmission and the measured time of arrival TOA of the ranging code at the receiver, the time of flight TOF required by the ranging code to propagate from the satellite to the receiver is determined. By multiplying this TOF with the speed of light, it is converted to the distance between the receiver and the respective satellite. The computed distance between a specific satellite and a receiver is called pseudo-range, because the general GPS time is not accurately known in the receiver. Usually, the receiver calculates the accurate time of arrival of a ranging code based on some initial estimate, and the more accurate the initial time estimate is, the more efficient are position and accurate time calculations. A reference GPS time can, but does not have to be provided to the receiver by a network.
The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the pseudo-ranges from a set of satellites. In order to be able to compute a position of a receiver in three dimensions and the time offset in the receiver clock, the signals from four different GPS satellite signals are required.
If navigation data are available on one of the receiver channels, the indication of the time of transmission comprised in a received signal can also be used in a time initialization for correcting a clock error in the receiver. In GPS, an initial time is needed for the positioning. For the initial time estimate, the average propagation time of satellite signal of around 0.070 seconds is added to the time of transmission of a ranging code extracted from the navigation information. The result is used as initial estimate of the time of arrival of a ranging code, which estimate lies within around 20 ms of the accurate time of arrival. The receiver then determines for different satellites the time at which a respective ranging code left the satellite. Using the initial estimate of the current time, the receiver forms pseudorange measurements as the time interval during which the respective ranging code was propagating from the satellite to the receiver either in seconds or in meters by scaling with the speed of light. After the position of the receiver has been calculated from the determined pseudoranges, the accurate time of arrival can then be calculated from standard GPS equations with an accuracy of 1 xcexcs.
However, in order to be able to make use of such a time initialization, the navigation data from a satellite signal is needed. Currently, most of the GPS receivers are designed for outdoor operations with good signal levels from satellites. Thus, only good propagation conditions ensure that the navigation data required for the described time initialization is available.
In bad propagation conditions, in contrast, it may not be possible to extract the navigation message accurately enough from received satellite signals, since a high bit-error rate and weak signal levels make a robust decoding of navigation bits impossible. Such bad propagation conditions, which are often given indoors, render the time initialization and the pseudorange measurements more difficult.
For those cases in which the standard time initialization methods cannot be applied since the navigation data are noisy, the time initialization process for the receiver can be performed by a time recovery method. Some known time recovery methods are based on the cross-correlation of a part of a tracked signal and an expected signal part to define the time of transmission.
Cross-correlation methods allow for determining the system time with an accuracy of one microsecond. It is a disadvantage of cross-correlation methods, however, that they require raw navigation data and a reference time with an accuracy of 6-30 seconds.
For the case that only submillisecond or sub-20 millisecond measurements of the time of transmission are available, e.g. from epoch and chip counts in the tracking unit of the receiver, it has been proposed to use a search technique over the possible GPS time which minimizes the sum of squared residuals or errors (SSE). Such a method has been presented by J. Syrjarinne in xe2x80x9cTime recovery through fusion of inaccurate network timing assistance with GPS measurementxe2x80x9d, Proc. 3rd Int. Conference on Information Fusion, Paris, France, July 10-13 2000, Vol. II, pp.WeD5-3-WeD5-10, and in xe2x80x9cPossibilities for GPS Time recovery with GSM Network Assistancexe2x80x9d, in Proc. ION GPS 2000, Salt Lake City, Utah, USA, Sep. 19-22 2000].
Moreover, a 5-dimensional Least Square approach has been proposed to find the position and the time based only on submillisecond or sub-20 millisecond measurements of the time of transmission at the receiver.
Both approaches have the disadvantage, though, that they rely on at least 5 visible satellites for determining the position and the system time.
It is an object of the invention to enable an estimation of the most probable time of transmission of a signal part of a beacon signal in weak signal conditions, in which less than five beacons are visible at the receiver and in which navigation data cannot be read from the received signals.
This object is reached with a method for determining an accurate time of transmission of a signal part of a code modulated signal transmitted by a beacon of a positioning system and received by a receiver of the positioning system, which receiver tracks beacon signals with a tracking unit.
In a first step of the proposed method, a subcomponent of the time of transmission of at least two signal parts is obtained from the tracking unit of the receiver. The subcomponent, which is measured by the tracking unit, indicates the difference in time between a detected regularity in a beacon signal comprising the respective signal part and the respective signal part itself.
In a second step of the proposed method, a plurality of expected subcomponents are predicted for each of the signal parts. The prediction is based on available estimates of the time of arrival and of the time of flight of the respective signal part. It is to be noted that the estimates may be available indirectly. The estimate of the time of flight might be obtained e.g. from available estimates of the position of the receiver and of the position of the respective beacon. For each predicted subcomponent for one signal part, a different total error within a known interval of possible total errors is assumed. Such a known interval can be known implicitly with known possible maximum errors for the estimates employed for determining the subcomponents.
In a third step of the proposed method, the combined difference between the measured and the expected subcomponent for each of the at least two signal parts is determined for each of the expected subcomponents separately. Thus, a combined difference is determined for each of the error values.
In a last step of the proposed method, the estimated time of transmission of at least one signal part is calculated. The calculation is taking into account the total error resulting in the smallest combined difference between an measured and an expected subcomponent.
The object of the invention is also reached with a receiver comprising means for receiving and tracking signals from at least one beacon and processing means for realizing the proposed method.
The object of the invention is further reached with a positioning system comprising a receiver and at least one network element of a network. This network may be a mobile communication network or any other network. The receiver again comprises means for receiving and tracking signals from at least one beacon and processing means for realizing the steps of the proposed method. In addition, the receiver comprises means for communicating with the network.
Finally, the object of the invention is reached according to the invention with a positioning system, in which the steps of the proposed method are realized by a processing unit of the system which is external to a receiver of the system. The receiver includes in this case means for receiving and tracking signals from at least one beacon and means for providing received and tracked beacon signals to the processing unit. The processing unit can also include other functions. It can be given e.g. by a mobile station to which the receiver is connected and which is able to communicate with a mobile communication network for receiving pieces of information. It can also be given by a network element of a network, in which required assistance data are available.
The invention proceeds from the consideration that the tracking unit of a receiver will often even be able to track received beacon signals of which contained information cannot be decoded. In such beacons signals, the tracking unit is further usually able to detect the place of a signal part, e.g. of the ranging code, of which the time of transmission is to be determined. Since the tracking is based on the codes which are repeated in known epochs, the tracking unit is at least able to count the epoch edges of any tracked signal. Sometimes, the tracking unit will in addition be able to detect bit edges in the signal. Thus, there exists an exact measurement for a subcomponent of the time of transmission of a signal part. This subcomponent determines in particular the time difference between a detected and time-stamped epoch or bit edge in the signal and the time of arrival of this signal part, i.e. the subcomponent determines the relative position of the signal part in a beacon signal. This time difference is the same as the time difference between the time of transmission of the detected epoch or bit edge and the time of transmission of the signal part. Such a time difference can be determined reliably in the tracking unit by an integer and fractional chip count and also by an epoch count if a bit edge is found. In GPS signals, for instance, the bit edges change every 20 ms and the epochs change every millisecond. The time subcomponent can thus be in particular either a submillisecond component or a sub 20 millisecond component.
The invention proceeds further from the consideration that the time of transmission of a detected part of a tracked beacon signal can be estimated, in case an initial reference time at the receiver, an initial reference position of the receiver, and an estimated time of flight of the signal required for propagating between the beacon and the receiver is available at the receiver.
Usually, also the maximum possible error is known for these estimates, thus the time of transmission can be determined with a total error lying within a known error interval.
Finally, the invention proceeds from the consideration that the subcomponent of the time of transmission of a part of a tracked beacon signal can be estimated separately based on the same time estimates as the entire time of transmission with the same known maximum total error. Thus, the difference between the measured subcomponent and an the estimated subcomponent can be determined for each or for selected ones of the possible total errors. In order to obtain a reliable result, the difference for each total error should be determined as combined difference for at least two different signal parts. The resulting minimum difference can be assumed to be associated to the correct total error.
This correct total error can then be employed for determining a quite accurate time of transmission of at least one signal part of a tracked beacon signal.
The invention is thus based basically on the equivalent of a cost function, which is minimized. Since the time subcomponents have a known scale, the cost function can be set up in a way that is has discrete time intervals. This restricts the evaluation of different possible error values to a limited, and still comprehensive number.
It is an advantage of the invention that the time of transmission of a signal part can be determined with an accuracy of one second by a single processing iteration at the receiver, while requiring signals from less than 5 beacons. The accuracy of the reference time can be several minutes, e.g. 5-10 minutes or more, and the reference position has to be known only with an accuracy of about 30 km, or even more.
It is further an advantage of the invention that the determined time of transmission of a signal part originating from one beacon can be used for determining the time of transmission of signals parts originating from other beacons. The time of transmission of other beacons can be determined e.g. with an accuracy of xc2x110 to xc2x1100 chips depending on the accuracy of the reference position. Thus, the invention can be used for a fast and more sensitive re-acquisition.
It is also an advantage of the invention that the navigation data in the received signal parts does not have to be available for determining a time of transmission. Further, available parameters employed in the calculations of the method according to the invention, e.g. reference time and reference position, do not have to be as accurate as for the known cross-correlation methods.
Preferred embodiments of the invention become apparent from the subclaims.
In a first preferred embodiment of the invention, the at least two signal parts originate from a single beacon. They are thus comprised in a single beacon signal, but are transmitted at different instances of time. This embodiment of the invention has the advantage over the state of the art that only a single beacon has to be visible for the receiver in order to be able to perform a time-recovery.
In a second preferred embodiment of the invention, the at least two signal parts are comprised in beacon signals originating from at least two different beacons. Advantageously, though not necessarily, the different signal parts are received by the receiver at the same instant of time. This embodiment has the advantage over the state of the art that as a minimum, only signals from two beacons are required instead of signals from five beacons.
The features of these two embodiments can also be combined to a further advantageous embodiment of the invention.
The combined differences between measured and expected time subcomponents can be determined in any suitable way. The differences may be calculated for instance as the sum of the absolute differences resulting for each of the at least two signal parts for a specific error value, or as the sum of the squared differences resulting for each of the at least two signal parts for a specific error value. The search for the minimum of the combined difference between the measured and the expected time subcomponents could be made particularly fast by using an integer least squares method.
The calculation of the time of transmission of a signal part may comprise as well a known correction value, which compensates e.g. for atmospheric influences, the current clock error of the beacon etc. Such a correction value may be determined in particular based on available navigation data.
The determined time of transmission of a signal part can be employed in particular for determining the time of arrival of this signal part, and thus for determining the accurate system time at the receiver in order to correct the local clock error. The time of arrival of a signal part can be determined like the time of transmission with an accuracy of one second. The accuracy of the receiver time will thus increase for example by 300 times, when proceeding from a reference time having an accuracy of five minutes to a corrected receiver time having an accuracy of one second.
The corrected receiver time can then be employed e.g. for determining the accurate time of transmission of signal parts from other beacons and for determining the exact position of the receiver from calculated pseudoranges between the receiver and the respective beacons as explained above.
The corrected receiver time can also be employed as a basis for a further refinement of the receiver time with another method. For instance, a known cross-correlation method could be based on the corrected receiver time for determining a particularly accurate system time at the receiver.
In case the receiver is part of a positioning system comprising a network element of a network, the processing in the receiver can be supported by the communication network. The network can provide e.g. the required time estimates, an information on the maximum possible errors of estimates and/or other assistance data. The network element comprises to this end means for providing the receiver with assistance data. As mentioned above, the network providing assistance data can be a mobile communication network, but it can also be any other kind of network which is capable of providing assistance data via a network element, e.g. via a DGPS (Differential Global Positioning system) station.
It is to be noted, though, that all required assistance data can also be available within the receiver itself. A receiver may for example be able to operate as a standalone receiver, which receives and decodes the navigation message in received beacon signals, initializes the time, etc., as long as it is situated outside. Therefore, the receiver is in possession of the necessary orbital parameters, e.g. ephemeris, and the internal clock is calibrated and will be quite accurate for some time. Also the receiver position is likely not to change by more than 30 km for some time. If the receiver is then moved e.g. into a building in which the navigation message of received beacon signals cannot be decoded, the method according to the invention can be thus used based on the available data within the receiver and without external assistance data.
Preferably, though not necessarily, the method according to the invention is implemented as software.
The beacons can be in particular, though not exclusively, satellites or a base stations of a network.
Preferably, though not necessarily, the receiver is a GPS receiver and the beacons are a GPS space vehicles.